1. Field of the Invention
This invention relates to changeover type variable magnification optical systems, and more particularly to changeover type variable magnification optical systems with an auxiliary lens removably located on the image side of a master lens assembly to decrease the focal length of the entire system. Still more particularly it relates to the increasing the magnification change ratio with the limitation of the size to a minimum, while still maintaining high stability of good performance against the change of magnification to be achieved so that the optical system is suited to photographic cameras or video cameras.
2. Description of the Related Art
There have been proposed a wide variety of variable magnification optical systems of the changeover type using an auxiliary lens, or so-called conversion lens, upon releasable attachment to the master lens assembly in front, or in rear, thereof to change the focal length of the entire system, while maintaining constant the position of the focal plane of the entire system. Of these, the front attachment of the conversion lens to the master lens assembly, or the so-called front conversion method, tends to increase the diameter of the conversion lens. Therefore, the use of it has made it difficult to achieve minimization of the size of the entire system.
The rear attachment of the conversion lens to the master lens assembly, or the so-called rear conversion method, on the other hand, though being more advantageous at reducing the lens diameter than the front conversion method, requires an increase in the necessary number of lens elements. Otherwise, the difficulty of aberration correction would increase. Hence, the use of it has tended to increase the length of the entire lens system in the longitudinal direction.
In Japanese Laid-Open Patent Application No. SHO 59-29214, as the rear conversion lens, use is made of a positive lens for the purpose of changing the focal length of the entire system to a shorter value. Thus, it claims a wide angle conversion lens.
The optical system employing such a wide angle conversion lens has, however, its ratio of magnification change set to as small a value of less than 2. Also, to attach the conversion lens, the master lens assembly must be positioned ahead. Hence, this conventional optical system tends to increase in size.
In order to heighten the ratio of magnification change with minimizing of the size of the entire system, it is in the general case that the refractive power of the conversion lens is strengthened. Yet, this method has a drawback that when the conversion lens is used, the amount of aberrations produced increases. Particularly spherical aberration and coma are affected largely. As the Petzval sum also increases, the difficulty of properly correcting the curvature of field increases.